1. Field of the invention
The invention relates to electrostatic painting apparatus comprising: a support which is movable at high speed through the intermediary of a least one drive member and which carries a pneumatic sprayer with a nozzle supplied with paint and pressurized gas through pneumatic valve means and means for electrostatically charging the sprayed jet, raised to a high voltage; a paint feed assembly controlled by air pressure and comprising a paint pressure regulator and a discharge valve; gas pressure regulating means for the sprayer and the pressure regulator, electrically controlled; electrically controlled pneumatic valves for controlling the discharge valve of the feed assembly and the valve means of the regulator; and programmed logic means adapted to define operating sequences.
2. Description of the prior art
Pneumatic sprayers are usually of two types designated round jet and flat jet. Round jet sprayers, utilized primarily for painting objects featuring openings, generally comprise a paint nozzle on the axis of a sprayer chamber into which open two pressurized gas distributors, the first of which is axial and causes the actual spraying and the second of which is tangential to generate a vortex.
Flat jet sprayers are designed primarily for painting large surfaces, such as automobile bodies, and also comprise a paint nozzle and an axial first pressurized gas distributor which surrounds the nozzle and a second pressurized gas distributor with oblique passages converging towards the axis of the sprayed jet, from either side thereof, so as to flatten the jet.
Note that for both types of sprayer the axial first pressurized gas distributor primarily conditions the fineness of the spray whereas the second, which is oblique relative to the axis of the nozzle, primarily conditions the shape of the sprayed jet (vortex aperture or flattening of the jet).
Nevertheless, there are interractions between the pressure at which the paint is delivered to the nozzle, the pressure of the gas supplied to the first distributor and that to the second distributor, to determine the discharge rate of paint from the sprayer and the shape of the jet, for adequate spraying quality. It will be understood that since the quality of spraying is a condition with which compliance is mandatory, there correspond to a pair of output parameters (paint discharge rate and jet shape) which relate to with the part which is to be painted, three input parameters: the pressure at which paint is admitted to the nozzle, the pressure of the gas supplied to the first distributor and the pressure of the gas supplied to the second distributor. Where relatively small quantities of parts are to be painted, these parameters are adjusted by an operator. For larger quantities correspondences are defined between the output and input parameters and recorded in the form of algorithms by means of which the input parameters may be obtained by entering the required output parameters.
French Pat. No. 1,537,997 describes a sprayer in which the three input parameters are modified by manipulating a single member which adjusts conjointly the fluid passage cross-sections.
As has already been implied, electrostatic painting aparatus for mass production lines must be able to work at the same rate as the line and be adaptable to the specific conditions for painting parts. In particular, it must be possible to adjust the sprayer in mid-cycle to modify the shape of the jet, the paint discharge rate and possibly the high voltage (in the case of hollow parts, for example). Also, it must be possible to shut off and restart the sprayer during each cycle, implying on/off valve means controlling the admission of fluids to the nozzle. All these operations may be controled according to predefined operating sequences by a microprocessor-based process control computer.
Incidentally, these operations form part of more extensive operating sequences, including for example adjustments concerning the travel of the support over a guide, displacement speed and change of direction point, in particular. In certain cases, displacements of the guide must be added to that of the support, in order to accompany a moving part or to dip into a cavity, for example. If the support is mounted on a multiple axis robot, an arrangement which is currently in widespread use, the operating sequence encompasses control of the robot relative to these various axes. Note that movements of the sprayer, in correspondence with the operating conditions, govern the adjustments specific to the sprayer: paint discharge rate, jet shape, high voltage.
Painting apparatus for mass production lines also incorporates color change sub-sequences which are commanded between the cycles for painting two consecutive parts. The color change processes, known per se, involve rinsing with a solvent to avoid one color being polluted by the preceding color.
The various aspects of automation of electrostatic painting apparatus for mass production lines have often been considered separately, the interface units between the painting apparatus and the controlling computer being progressively added or substituted for less effective units on older apparatus. It has been realized that the general organization of painting apparatus should take into account the response times of these interface units, these times including transmission times over the connecting lines.
These times are determined by the speed of movement of the support which carries the sprayer. On a reciprocating carriage the displacement speed is usually around one meter per second. In reciprocating movement, direction changes involving accelerations of 10 ms.sup.-2 last 0.2 seconds. The sprayer adjustments must also be carried out in less than 0.2 seconds, representing a travel of 0.2 m. The time to output a digital control signal is expressed in microseconds; there is no problem from this point of view. Slaving the position of a fluid pressure adjusting member to an electrical control signal involves time constants on the order of one hundredth of a second, which is entirely acceptable. However, the transmission of a fluid pressure along a pipe is significantly slower, depending on the length and cross-section of the fluid passage.
With multiple axis robots, the tolerable adjustment times are at least as short as in the case of a reciprocating carriage.
In a similar manner, adjusting the high voltage applied to the members for electrostatically charging the jet involves a time constant which, broadly speaking, is defined by the impedance of the supply as seen by the charging members and the capacitance as seen by the supply. To give an idea of the orders of magnitude involved, to obtain in 0.1 seconds a voltage of 60 kilovolts with a supply producing 60 microamperes, the maximum capacitance is 100 picofarads. The self-capacitance of a shielded connecting cable rated at 60 kilovolts is routinely between 30 and 40 picofarads per meter.
If the support were made to carry all the electrically controlled adjuster devices and the pneumatically controlled actuator devices, in order to reduce the distance of these devices from the sprayer and the control time constants, the weight carried by the support would be incompatible with the accelerations which must be applied to it, unless the support, its guides and the drive means which actuate it were reinforced. Such reinforcing arrangements are inevitably costly and bulky, and are often of an illusory nature; when the weight of the active members becomes negligible in comparison with that of the accessory and supporting members, the gain resulting from such reinforcement is virtually all absorbed by the resulting increase in weight due to the reinforcement.
An object of the invention is a sprayer apparatus in which the weight carried by the mobile support is reduced to a minimum although the time to execute adjustments is compatible with carrying out the same while in operation.